Solar sail
A solar sail is a model of spacecraft that uses the Stellar Wind for propulsion. The stellar wind (or the Solar wind in case of our sun) is a continuous stream of charged particles ejected by the Sun. Used properly, it has the ability to push an object. The only successful solar spacecraft deployed is Ikaros, built by JAXA, the Japanese Space Agency. The ship flew to a Venus flyby and is still functional. Efficiency Ikaros has a surface of 196 square meters. On this surface, the solar wind pressure is 1.12 mN. A square solar sail (800/800 m) can receive a force of 5 N (roughly 0.5 kg force). The force is very weak, but it requires no propellant. By comparison, NASA's Dawn spacecraft used ion engines to travel to Vesta and Ceres. The three ion engines offered a thrust of 0.09 mN each. It can be questionable if building a large solar sail is cheaper or not then the use of ion engines. Advantages A solar sail requires no energy to move (even if some propellant might be required for attitude control, see below). It is an useful way to ship goods from an Inner Planet to an Outer Planet, especially in case of stars with a powerful Stellar Wind. It is also possible to use a solar sail to decelerate and move inwards. However, such a maneuver requires fuel consumption for attitude control. A third usage of solar sails is for precision guidance. Messenger spacecraft used its solar panels as solar sails to precisely adjust its trajectory towards Mercury. A solar sail could become very efficient when flying close to a star. Its efficiency decreases dramatically when heading to the outer reaches of a solar system. However, the Delta-v required to change orbit near an inner planet is also big. Sending a ship from Mercury's orbit to Pluto's requires far more energy then sending a ship from Pluto's orbit to Mercury's. Concerns The major disadvantage of a solar sail is its size. If we want to build a solar ferry between Earth and Mars, it would require to be hundreds of km wide. At that size, the sail would require very strong materials to hold itself, adding extra mass. As experienced by Ikarus, a major concern is attitude control. Solar wind will tend to make the sail rotate and reach an equilibrium position, where it is less impacted by the solar wind. There are concepts of a parachute-like sail, which would be maintained opened by the wind itself. A spherical sail could also be useful. Ikarus used liquid crystal devices, which could turn transparent or opaque, controlling the attitude. A solar sail can efficiently push an object away from the Sun. To push a ship towards an inner planet, the sail needs to be tilted in an angle, reflecting the wind. The sail will constantly tend to rotate and will require an auxiliary propulsion system or reaction wheels for attitude control. Given the size of a sail, maintaining attitude is far more difficult then in case of other space ships. Ikarus managed to maintain its attitude with the help of its liquid crystals, constantly turning them transparent or opaque. Because solar sails are big, they are more exposed to impacts. Conclusion Solar sails could be an efficient way to transport cargo inside a solar system. They cannot be used for very large ships. Their use for passenger ships is questionable.